A transmitter-receiver combination utilizing radio frequency coupling has frequently been proposed for operating and controlling various systems, such as alarms for detecting burglaries, fires, thefts, and the like. Equally, such systems are useful for monitoring liquid levels in tank farms, medical dysfunctions, interoffice communications, and other such applications. Unfortunately, the range of such equipment, until now, has been seriously limited because, when the transmitter or receiver is personally carried, the transmission power is small, and thus the signal is quite weak. Added to this, if the frequency band used obviates the necessity for license, the band has a high density of background noise in combination with the encoded signal. Thus, a relatively weak signal and a low ratio of signal to noise, make it difficult to amplify, detect the presence of, and analyze, an encoded signal in any practical way of coupling a transmitter to a receiver over any appreciable distance, such as 1,000 feet or so using equipment of a reasonable cost.
In other words, the cost effectiveness of a radio system in which there are relatively weak signals, transmitted in a crowded frequency band, has been thus far beyond the capability and understanding of the art.
While some radio frequency coupling devices have been proposed, their range is substantially limited, or they require, in Canada, a Communications Canada license. What is required is a radio transmission control device which does not require a licsense to operate; is extremely sensitive to achieve the necessary range; but whose sensitivity is not defeated by background noise which tends to obscure relatively weak signals, particularly when the signal is greatly amplified.